The present invention is an improvement to a driving circuit for a series or compound motor. The improvement is constituted by two sets of series excitation field windings, one end of each of the series excitation windings being connected, with the same polarity, jointly to the armature, and at least one of the series excitation field windings being connected by means of diodes to receive power from two different power sources. The two parallel series excitation field windings replace the conventional single series excitation field windings. By means of this innovative design, the magnetic field of the series excitation motor can be better controlled by causing a decrease in the excitation current from one of the excitation windings to be offset by an increase in the other and thereby reduce the drop in efficiency of a conventional series excitation motor under light load or overload conditions.